The present invention relates to a process for coupling low molecular weight telomers of chlorotrifluoroethylene, hereinafter designated as "CTFE", to produce commercially useful products.
CTFE telomers are saturated, low molecular weight polymers, typically of general formula CCl.sub.3 (CF.sub.2 CClF).sub.n Cl, where n, the molecular number or chain length (the number of repeating units in the telomer chain) is in the range of 1 to 20.
Various methods of preparing such CTFE telomers are known in the prior art and have been practiced commercially for many years. An article by William T. Miller, Jr. et al in Industrial and Engineering Chemistry, pages 333-337 (1947), entitled "Low Polymers of Chlorotrifluoroethylene", describes a process for producing low molecular weight polymers of CTFE by carrying out the polymerization in a solution of chloroform using benzoyl peroxides as a polymerization promoter. Other solvents disclosed in the reference as being useful for this purpose include carbon tetrachloride and tetrachloroethylene. The solution is heated in a pressure vessel for 13/4 hours at 100.degree. C., and the unreacted CTFE monomer and chloroform are removed by distillation, leaving a "crude" telomer of general formula CHCl.sub.2 (CF.sub.2 CClF).sub.n Cl, which can be further heated and distilled to yield products ranging from a light oil to a semi-solid wax or grease.
Another process which has been developed for producing low molecular weight CTFE polymers is described in U.S. Pat. No. 2,788,375, issued Apr. 9, 1957. This process comprises reacting CTFE with a saturated organic bromo compound, such as bromotrichloromethane, in the presence of actinic light in a de-oxygenated system to obtain saturated bromopolychlorofluoro compounds containing one or more CTFE units per molecule. These saturated bromopolychlorofluoro compounds can then be converted to corresponding polychlorofluoro compounds by reaction with chlorine, and subsequently reacted with fluorinating agents to yield more highly fluorinated products.
A more recent development in this field is described in a series of articles by Y. Peitrasanta et al entitled "Telomerization by Redox Catalysis" appearing in the European Polymer Journal, Vol. 12 (1976). This technology involves the reaction of a chlorinated telogen, such as carbon tetrachloride, with CTFE in the presence of benzoin and a suitable redox catalyst, such as ferric chloride hexahydrate (FeCl.sub.3.sup.. 6H.sub.2 O). The telomerization reaction is suitably carried out in acetonitrile which is a common solvent for the reactants and catalysts. The telomerization reaction can be illustrated as follows: ##STR1##
Such telomers can be fluorinated with known fluorinating agents, such as cobalt trifluoride, chlorine trifluoride or hydrogen fluoride to produce products which have a higher degree of stability, and are therefore of more commercial importance. Such fluorination processes are disclosed in British Patent Nos. 712,184 and 761,053, U.S. Pat. Nos. 2,636,908, and 2,886,607.
There are certain disadvantages inherent in all the methods used to produce CTFE telomers which are suitable for particular applications. For example, the production of hydraulic fluids or light oils typically requires telomers having a chain length of from 3 to 6 carbon atoms, while the production of greases or waxes requires longer chain length telomers, i.e. those having 12 or more carbon atoms in the chain. For such applications, it is desirable to maximize the yield of products having the specific characteristics as defined by the end user's product specifications. This necessitates a fairly narrow range of molecular weights in most instances. However, commercial processes actually produce a distribution of telomers having a comparatively wide range of molecular weights, including low molecular weight telomers, i.e. those having molecular numbers of 1 and 2, which due to their volatility have no commercial value.
In U.S. Pat. No. 4,307,259, a process is described for coupling trimers which are prepared by the gas phase photochlorination of bromotrifluoroethylene. The trimers, which are obtained by fractional distillation of the polymerization products, contain a single bromine atom on a single end group of each trimer. The trimers are reacted with one mole of zinc per two moles of trimer in the presence of acetic anhydride forming ZnBr.sub.2, and a coupled, fully saturated product having twelve carbon atoms.
It is therefore a principle object of this invention to provide a process for coupling low molecular weight telomers of CTFE to produce unsaturated compositions of a higher molecular weight which can be further fluorinated to yield a stabilized product. It is further the object of this invention to provide a process for utilizing the lighter weight telomers from a CTFE telomerization process to produce higher molecular weight telomers useful as hydraulic fluids.